Ironically, with X-Com at least, you want processor taxing. X-Com was released back when there were only a few processor speeds, so the game will run as fast as the processor allows. i.e. At 2.8 GHz, the game is completely unplayable. I believe to play X-Com on a modern PC, it must either be run in a virtual environment or with a processor time occupier. I'm not sure how Steam handles it, but I've used the time occupier and it works well. Mind you, that was running in Windows natively. I would ask on the Steam forums or their tech support. Parallels or Fusion might not take well to the tricks required to run such a program.